After importing a .fbx model in blender it's not smooth with visible faces in viewport as well as render
I haven't used Fusion, but I know that the models in that software are geometric representations for mathematically-based models of designs to be used in real life. This means the geometry is automatically generated depending on things such as display resolution, zoom on the model, and model complexity among other things.

When you convert it into an FBX model, Fusion has to decide how to compress this mathematically-based model into an actual geometric model. I believe that unless Fusion doesn't have the setting to increase the resolution of exported models (which I doubt), then this issue isn't necessarily a problem for Blender.

Nonetheless, if you cannot make model smoother by using the subsurf modifier, make sure the modifier is visible to both the camera and the viewport (the eye and camera symbols in the modifier tab should be selected):



Otherwise, I'd suggest this:


Make sure the object is actually a mesh and not something else like curves possibly (if not convert it into one).
(optional, but might make selection easier) Use Alt+J to turn triangle faces into square faces.
Select the hard edges of the model and use Shift+E to add creases.
Use the Subsurf Modifier or smooth subdivide to smooth the mesh.